Science Questions

How do farmers propagate seedless crops?

Sun, 6th Dec 2009

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Question

Gary Staab asked:

How do farmers propagate seedless crops?

Answer

We put this question to Stephen Tomkins, from Homerton College, Cambridge, and Jennifer Schultz Nelson, Horticulture Educator with the University of Illinois:

Wild type bananaStephen -   You need to know something about why they have no seeds and that’s because they're often triploids.  That is, they've got three sets of chromosomes and you and me have two.  And when it comes to sexual cell divisions in meiosis, they can’t organize their chromosomes properly and they fail, and therefore, they have no babies.  So the question that you want answered is how do you get to a triploid fruit?  And the answer to that is a little bit complicated.  Most plants have two sets of genes, two sets of chromosomes and they divide equally when cell divisions take place.  But plants are very tolerant of having multiple sets and that’s called polyploidy.  Polyploid plants can be triploid, tetraploid, pentaploid, and up the numbers go!  And it doesn’t often badly affect the viability of the plant.  They grow.  They photosynthesize.  They can produce fruits.  But if they have odd numbers of chromosomes, they are stuffed.  They can't have any babies at all.  And that happens to the triploids and the pentaploids because you can't divide odd numbers evenly.  If you want to multiply up that plant, you can do it by cloning or vegetative propagation.  And that’s an important thing for people setting out to grow large orchards of triploid fruit.

Diana -   So you can propagate plants which are gameteless so they make seedless fruit.  And here’s a little more on how it’s done.

Jennifer -   When humans get involved, they try to manipulate things and they can do things like treat a plant with a chemical called ‘colchicine’ which disrupts the meiotic process.  So when the plant is producing pollen grains and ovules, they will produce a gamete that has double the amount of chromosomes.  So then when it is pollinated by the original plant that provides one copy of chromosomes, the result is a triploid plant.  We talk about cloning being a really modern method, but actually, horticulturists have been using it for quite some time.  Any time you're taking a cutting of a plant and rooting it using different plant hormones or simply sticking it in a glass of water, watching the roots grow and then planting that plant, that’s cloning, also called ‘vegetative propagation.’  I actually found some information that when you talk about polyploidy in plants and it sounds like a really rare thing, but it’s actually not that rare.  Scientists think that anywhere from 30 to 70% of angiosperms from the plant kingdom are polyploidy and polyploidy plants tend to be larger.  And so, maybe they're more competitive in nature and so that might be some advantage for that to be selected for in nature.

Diana -   Triploid plants can be reproduced using cloning and vegetative propagation as well as chemical mutagens like ‘colchicine.’  But many farms will grow the fertile parent plants nearby so that more seedless offspring can be produced later.

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Seedless crops are planted by taking cuttings from plants. In the case of fruits, these cuttings are usually referred to as 'sticks'. Don_1, Tue, 1st Dec 2009

I think bananas do occasionally make seeds - but rarely.

c chris, Tue, 1st Dec 2009



The answer to your question begins with the chromosome. A chromosome is an organized structure of DNA and protein that is found in cells. We have about triple the number of chromosomes as human beings than the (simpler) platypus.

Fruit development normally begins when one or more egg cells in the ovular (‘egg like’) compartment of the flower are fertilized by sperm nuclei from pollen.

In some plants, however, fruit develops without fertilization, a phenomenon known as parthenocarpy.

Parthenocarpic fruit has advantages over seeded fruit: longer shelf life and greater consumer appeal.

Watermelons and bananas are probably the two most famous examples of seedless technology that exploit this phenomenon.

One way to make seedless watermelons is to produce triploid seed (three copies of chromosomes instead of the usual two).

As in the case of seedless bananas, triploid watermelons cannot produce functional seed, but they still develop good fruit through parthenocarpy. Plant breeders produce triploid seed by crossing a normal dparent with a tetraploid parent (a parent that has four sets of chromosomes).




And, of course, horticulturalists are working actively on using genetic engineering to make plants seedless. I’ll leave it to the experts in that field to seed us with the relevant info on that…….


Have a nice week guys! Shibs, Wed, 2nd Dec 2009

Some plants can be propagated with cuttings but some, like bananas are cloned. Farmers have been cloning plants for quite a long time. Cloning animals is quite a lot harder and has only recently been adapted for animals.



Bananas do produce seeds normally. The seeds are in the meat of the fruit and in wild bananas they are large and very hard, quite inedible. When you eat a seeded banana you got a mouthful of seeds with every bite. "Seedless" bananas do have seeds but the plant has been bread in such a way that today these seeds are tiny. If you slice a banana you can see the seeds as tiny dark flecks in the fruit. They are so small you don't even notice them. mountaineirc1969, Wed, 2nd Dec 2009

Mmm, but those banana seeds in seedless bananas are not fertile / viable, Eric.

Banana plants are all clones of each other, produced, I think, from root sprouts. Consequently they are all genetically identical and vulnerable to the same pests, especially fungi; Panama disease is one such example, which did for the Gros Michel banana; the Cavendish is similarly threatened by another fungal pathogen although I cannot recall the name of this one. chris, Wed, 2nd Dec 2009



I know that. I didn't mention it as I assumed everyone knew it. mountaineirc1969, Wed, 2nd Dec 2009

Did anyone else spot the contradiction in this "Banana plants are all clones of each other, produced, I think, from root sprouts. Consequently they are all genetically identical and vulnerable to the same pests, especially fungi; Panama disease is one such example, which did for the Gros Michel banana; the Cavendish is similarly threatened by another fungal pathogen "?
Bored chemist, Wed, 2nd Dec 2009

I don't see the problem. What is wrong? chris, Wed, 2nd Dec 2009

Some plants are naturally seedless but then we have the gene modified types.
http://fbae.org/2009/FBAE/website/our-position-gene-use-restriction-technologies.html

Monsanto Whistleblower Says Genetically Engineered Crops May Cause Disease.
http://www.globalresearch.ca/index.php?aid=3912&context=va

And finally.
http://en.wikipedia.org/wiki/Monsanto yor_on, Fri, 4th Dec 2009


If all bananas are clones of each other how can there be different varieties? Bored chemist, Sat, 5th Dec 2009

The bananas we eat are all clones and that's why the Cavendish is threatened.

But that's not the same as saying that it's not possible to manipulate genetically distinct (weakly or inedibly-fruiting) banana plants to yield novel fruiting varieties (hence different cultivars) that are resistant.

Chris chris, Sat, 5th Dec 2009

Incidentally, the Cavendish isn't threatened because it's a clone, but because it's a monoculture. Being a clone certainly doesn't help, but it's not the root of the problem (if you forgive the pun).



***my apologies - I thought I was quoting this article to reply to it and then discovered I'd edited it instead. Sorry. Chris *** Bored chemist, Sun, 6th Dec 2009

Hi,

Regarding the reproductive abilities of seedless fruits:

Many commercial fruit plants can be reproduced readily by cloning. In fact for some kind of fruits that is the norm whether the fruits have seeds or not.

If you planted seeds from your favorite variety of apple, for example, the resulting trees wouldn't produce the same apples again, but instead they'd each produce their own unique apples. Kind of like your kids may remind people of you, while not really being that much like you.

Usually these unique apple trees are not helpful commercially because the growers want to sell recogizeable names of apples that are currently popular or famous, and do so in bulk.. So what the growers do is they graft cuttings from the parent plant onto bare root stock and let them grow into clone copies of the parent tree. For any given variety you might see at the market, all the thousands of apple trees that produce those apples all got cloned from one original tree (or from later clones of that one original tree). What happened to that one original tree's seeds are pretty much irrelevant! Same with the seeds of the clones.

I'm sure there are other ways to do it too, that would work better on other kinds of fruit plants. For example I have read that some of the annuals that are seedless are hybrids and so the seeds to plant more are obtained by crossing 2 other varities. But on the apple farm, grafting is how it's done!

Hope this helps,

--Kaas Baichtal
Bayfield Apple Co
http://www.bayfieldapple.com/

kbaichtal, Mon, 7th Dec 2009

kb - Re Apples

I have a book on the history of apples, and you are correct. For instance, all Red Delicious Apples are descended from one single tree found on a farm in the US someplace.  THAT tree was almost certainly the result of a random seed planted by accident through bird dispersion.

However, not all Red Delicious Apples look or taste the same. I know of only two ways this is possible. First, apple trees sometimes sprout a 'sport growth' among their own limbs that show noticeable differentiation from the rest of the tree. The other way is to cross bred it with other apples.

FYI, Johny Apple Seed was a real person, and had a real purpose in life. Back in those days it was easier to plant large numbers of a large variety of apple seeds in the hopes of growing one with particular advantageous properties then to slowly cross pollinate existing varieties. At least thats what the story is....

PS: I grew up on a farm with lots of apple trees that did not include Red Delicious. These things propogated by seed all over the place. One of the shrub like trees produced a deep red apple much like a Red Delicious, but with a deeper crispier rich taste. To this day I wonder if we missed out on he Next Big Apple. litespeed, Mon, 4th Jan 2010

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